Process for manufacturing high grades of specialty electrical steels

ABSTRACT

This is a process for the production of specialty electric steel, particularly grain oriented electrical steel, and more particularly, grain oriented silicon electrical steel. The steel can be formed starting from a thin slab. The process can relate to a product formation route which enables efficient production with better yield and wider process control tolerance. The method can be employed for producing specialty electrical steel utilizes cheaper inputs, less energy, combines and overlaps production process steps, improves yields and product uniformity. This can be accomplished by making it more tolerant to a wider range of process parameters.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of copending U.S. provisional patent application Ser. No. 61/703,701, filed Sep. 20, 2012, herein incorporated by reference.

BACKGROUND

This is a process for the production of specialty electric steel, particularly grain oriented electrical steel, and more particularly, grain oriented silicon electrical steel.

The prior art describes variations of the product and the process to make many variations of electrical steels. Patents issued to Hadfield starting in 1903 (such as U.S. Pat. Nos. 745,829; 836,762; 836,754; 836,755; 836,756) are among the earliest patents in this field. Such patents describe the magnetic performance of electrical steels and the composition for making electrical steels with methods using the technology available around the year 1900. Patents assigned to Armco Steel Corporation, Ohio and the General Electric Company, New York, from the year 1950 onward (such as U.S. Pat. Nos. 2,535,420; 2,599,340; 2,867,558) describe variations and improvements to the product and process to incorporate continuous manufacturing operations and improved process control.

Traditionally, electrical steels have been made by casting ingots or slabs that are 200-250 mm thick. In such processes large oriented grain growth is obtained in the final stages of the process at a step referred to High Temperature Anneal (HTA) where the steel strip is held at elevated temperatures of around 1200° C. for an extended period of time. In the HTA step certain chemical systems inhibit the growth of general or normal grains while allowing the large oriented grains to grow. These chemical systems are referred to as the “inhibitor system” for a given process. In the past, electrical steels have used one of two inhibitor systems, which are the (a) sulfide-manganese system, and (b) nitride-aluminum system.

The sulfide-manganese system has been known to result in high quality electrical steels but it has several major drawbacks. It requires high temperature reheat of the slab to re-dissolve the inhibitor species which tend to escape from the iron crystal grains when the slab is solidifying after casting. It also requires tight process control since sulfur has a high propensity to escape from the iron crystal grains. Moreover, when sulfur rich chemical species collect at the grain boundaries they cause the problem of red-shortness or hot-shortness which results in cracking and breakage of steel strips and results in yield loss.

The nitride-aluminum system has been used to make electrical steels with a lower reheat temperature of the slab. But since a thick slab (200-250 mm) takes a considerable time to solidify it still provides an environment in which the inhibitor species escape from the iron crystal grains. As such the process has a few drawbacks: 1) it still requires an energy intensive reheat step; and 2) the inhibitor species have a propensity to chemically combine with other impurities present in the steel and thus result in lower levels of inhibitors at the HTA step and also create new impurities that compromise the performance and properties of the finished electrical steel.

In the recent past attempts have been made to produce electrical steels using sulfide-manganese inhibitor systems along with thin slab casting technology which casts a slab from 20-80 mm (see for example U.S. Pat. No. 6,296,719). This offers the benefit of low energy consumption since the slab can be rolled to final gauge from a much smaller starting thickness. It also offers the benefit of obtaining favorable microstructure in the slab. But since it is based on sulfide-manganese inhibitor systems, the process still requires slab reheat to about 1300° C. and still is susceptible to the drawbacks which are characteristic of such systems.

SUMMARY

The subject process overcome all of the problems described above with respect to the prior art systems for producing electric steel. Furthermore, the present method and system is not based on sulfide-manganese inhibitor systems.

Contrarily, this process is for the production of specialty electric steel, particularly grain oriented electrical steel, and more particularly, grain oriented silicon electrical steel. The steel can be formed starting from a thin slab. The process can relate to a product formation route which enables efficient production with better yield and wider process control tolerance. The method can be employed for producing specialty electrical steel utilizes cheaper inputs, less energy, combines and overlaps production process steps, improves yields and product uniformity. This can be accomplished by making it more tolerant to a wider range of process parameters.

DESCRIPTION

The subject process for producing specialty electrical steel utilizes cheaper inputs, less energy, combines and overlaps production process steps, improves yields, and enhances product uniformity.

In an embodiment, a core can be provided which promotes flux magnification in an electromagnetic system. A core is whatever fills the space in a solenoid. In one embodiment an air core is provided, in another embodiment an iron-based core is provided, and in a further embodiment a core of a layered material is provided.

In an embodiment herein, an iron-based core is provided which promotes flux magnification in an electromagnetic system. In one embodiment, a soft iron-based core is provided which promotes the desired flux magnification in an electromagnetic system, and in a further embodiment an non-alloyed iron iron-based core is employed for that purpose.

In one embodiment, about four orders of magnitude less current is required to produce a magnetic field of given strength in a solenoid with the above-described iron-based core versus a solenoid with no core, another embodiment about three orders of magnitude less current is required to produce the above-described magnetic field, and in still another embodiment about two orders of magnitude less current is required to produce the above-described magnetic field.

The ideal properties of a core would be one in one embodiment that it magnetizes and demagnetizes instantaneously. This can be accomplished when current flows in spiral coils (solenoid) around the core in one direction the core is magnetized as the magnetic domains align with the magnetic field created in the solenoid. When the current flow changes direction, the direction of the magnetic field is reversed, and the core is demagnetized in the reversed direction.

In another embodiment an ideal property of the core is that it loses substantially no energy in the process, in a further embodiment that it maintains the above-described properties forever, and yet another embodiment that it is small in size. Electromagnetic systems can approach this ideal condition in one embodiment through a combination of core design and how it is powered on one hand, and in another embodiment by selecting relevant properties of the core on the other hand.

In an embodiment herein, a process is provided for making high grades of specialty steels commonly known as electrical steels. In a still a further embodiment, these electrical steels are employed for making iron-based core material.

In an embodiment, the subject steel product is formed starting from a thin slab. In one embodiment, the thin slab has a thickness of from about 1 mm, in another embodiment from about 8 mm, and in a further embodiment from about 10 mm, and in yet another embodiment up to a thickness of about 150 mm, in still another embodiment up to a thickness of about 140 mm, and in still a further embodiment up to a thickness of about 130 mm.

The process includes various steps. In a first step, steel can be melted and refined. In one embodiment, the steel is melted at a temperature of up to about 1500 degrees C., in another embodiment at a temperature of up to about 1450 degrees C., and in a further embodiment at a temperature of up to about 1600 degrees C.

In one embodiment, the refining step includes performing a chemical analysis of the molten steel, in another embodiment it includes identifying what must be removed by a chemical reaction and/or added to obtain the desired chemical analysis, and in a further embodiment it includes adding the requisite chemicals for reaction and/or addition to the molten steel. In another embodiment, a chemical analysis of the molten steel is performed which includes identifying which chemical components of the steel feed should be removed by conducting a chemical reaction and/or by adding a chemical component to obtain the desired chemical analysis, and then removing the requisite chemical components from the molten steel and/or adding the requisite chemicals to the molten steel.

Steel feed is in the molten state and the chemical composition adjusted as follows: in one embodiment the amount of Aluminum is up to about 0.5%, in another embodiment the amount of Nitrogen is up to about 0.05%, still another embodiment the amount of Manganese is up to about 0.3%, and in a still further embodiment the amount of Silicon is up to about 5%, by weight, all based on the total weight of the molten steel.

In a second step, a slab is cast from the molten steel. Various options of casting equipment can be employed. In an embodiment herein the cast steel thickness can be from about 1 mm up to 150 mm. In another embodiment, if the thickness of the cast steel is between about 1 mm to about 7 mm, then it is referred to as a strip and the process proceeds to step 5. In a further embodiment, if the cast thickness is between about 8 mm to about 150 mm, the process proceeds to step 3

In step 3, in one embodiment, the slab from step 2 is reheated to a temperature between about 1150 to 1400 degrees C. In another embodiment, the slab from step 2 is reheated to a temperature between about 1175 to 1350 degrees C. In a further embodiment, the slab from step 2 is reheated to a temperature between about 1200 to 1300 degrees C.

In step 4, the thickness of the slab from step 3 is reduced through, in one embodiment, a rolling mill, to form a hot band having a thickness of from about 1.0 mm to 2.5 mm. In an embodiment, the hot band exits the rolling mill at a temperature of about 950 to 1050 degrees C.

The hot band material from step 2 or 4 is treated in step 5. In one embodiment it is initially cooled. In yet another step initial cooling is conducted in the ambient air. In another embodiment initial cooling is conducted for about 5 to 7 seconds. In a further embodiment, the hot band is rapidly cooled. In another embodiment, rapid cooling is conducting by exposing the material to water. In a further embodiment rapid cooling is provided to a temperature of about 550 degrees C.

In step 6, in an embodiment, the hot band material from step 5 is annealed and normalized. In another embodiment, the material is annealed and normalized so that it is heated and cooled such that the physical micro-structures are more uniform. In a further embodiment, step 6 is conducted in an annealing chamber. In still another embodiment it is treated in a protective atmosphere of N₂. In still a further embodiment the materials is treated for about 5 to 7 minutes. In yet another embodiment, the material is treated at a temperature of from about 950 to about 1150 degrees C.

In step 7, in an embodiment, the material from step 6 is exposed to a surface treatment by a reactive material in a chamber. In another embodiment, the material is subject to a pickling treatment in the chamber. In a further embodiment, the surface treatment described above removes about 20 to 30 g/m² of a scale formed on the material. In still another embodiment, the scale is removed with up to about 5% by weight of SiO₂.

In an embodiment, the material from step 7 is reduced to about a 0.65 mm thickness in step 8. In another embodiment, the material from Step 7 at step 8 is reduced to a 0.63 mm thickness. In a further embodiment, the material from Step 7 at step 8 is reduced to a 0.48 mm thickness. In still another embodiment, the material from Step 7 at step 8 is reduced to a 0.33 mm thickness. In yet another embodiment, the thickness is reduced by cold rolling.

In an embodiment, the material from step 8 can be sent to an alternate step, namely 9.a, in a chamber where the material from step 8 is treated to form non-oriented grains. In another embodiment, an insulative coating is applied to the outer surface of the material from step 8. In a further embodiment, a coating that does not substantially conduct electricity is applied to the outer surface of the material from step 8. In still another embodiment, the insulating coating is a varnish. In still a further embodiment, the treatment in step 9.a is conducted for about 3 to 5 minutes. In an embodiment, the treatment in step 9.a is conducted at about 950 to 1050 degrees C. In one embodiment, the product of step 9.a is Non Oriented Electrical Steel

In step 9, in an embodiment herein, decarbonization and annealing is conducted in a chamber on the material from step 8. Decarbonization is the removal of Carbon from the material from step 8. In an embodiment, the material to be decarbonized is subjected to treated with Hydrogen gas and/or Water Vapor. In another embodiment, the material form step 8 is initially heated to a temperature of about 550 degrees C. In still another embodiment, the initially heated material is then heated to a temperature of about 850 degrees C. In the conditions describe above for decarbonization, the Hydrogen and/or Water Vapor is reacted with the Carbon in the steel to remove it therefrom and pass it into a gaseous state as Carbon Dioxide. In a further embodiment, the material from step 8 is treated in a protective atmosphere of N₂. In another embodiment, the protective atmosphere of N₂ treatment is conducted in a H₂O/H₂ atmosphere. In still a further embodiment, the H₂O/H₂ ratio is from about 0.05 to about 0.95. In still another embodiment, the treatment time is for about 7 to 8 minutes. In yet another embodiment the treatment temperature is from about 800 to 900 degrees C.

The thickness of the material from step 9, in an embodiment herein, can be reduced in a second cold rolling step of step 10. In one embodiment, the thickness is reduced to from about 0.16 mm, and in another embodiment to a thickness of from about 0.21 mm, and in another embodiment to a thickness of from about 0.25 mm, in a further embodiment to a thickness of from about 0.28 mm, in still another embodiment to a thickness of from about 0.33 mm, and up to from about 0.63 mm, in still a further embodiment up to from about 0.60 mm, and in yet another embodiment, up to from about 0.55 mm.

The material from step 9, in an embodiment herein, can also be treated in an additional step 9.b prior to step 10, in a chamber such that the exposed surface of the material is treated with liquefied ammonia. Ammonia comprises the chemical formula NH₃ which has no charge or is neutral. Ammonia which is treated with free radical hydrogen of chemical formula [H]+bonds with Ammonia to form NH3 [H]+ which has a positive charge and is called active ammonia. In another embodiment, the material from step 9 can be treated with activated ammonia. In a further embodiment, the material from step 9 is treated for from about 15 to 30 seconds. In still another embodiment, the material from step 9 is treated at a temperature of from about 700 to 800 degrees C. When the product is treated employing step 9.b, the final product from step 13 below is High Induction Electrical Steel.

The material from step 9, in an embodiment herein, is treated in a magnesia coating step 10 in a chamber to apply magnesia to keep strip from sticking in step 11 below. In another embodiment, the material from step 9 can form an initial insulative coating in coating and drying equipment. In a further embodiment, the initial insulative coating step is conducted at atmospheric pressure. In still another embodiment the initial insulative coating step is conducted for about 1 to 2 minutes. In still a further embodiment, the initial insulative coating step is conducted at a temperature of about 600 to 700 degrees C.

A annealing step 11 is conducted in a chamber, in an embodiment herein, where the material from step 10 forms large grains. In one embodiment, the large grains are oriented in the rolling direction. In another embodiment, the material is treated for at least about 130 hours, in still another embodiment for at least about 100 hours, and still a further embodiment for at least about 150 hours. In a further embodiment, the material is treated at a temperature of at least about 1200 degrees C., in still another embodiment it is treated at a temperature of at least about 1100 degrees C., and in yet a further embodiment it is treated at a temperature of at least about 1250 degrees C., to form large grains oriented in rolling direction and a secondary insulative coating is developed. In a further embodiment, a second insulative coating that is substantially non-conductive, and does not substantially conduct electricity, is applied to the outer surface of the material from step 10. In still another embodiment, the second insulating coating is a varnish.

In an embodiment, the material from step 11 is treated in a chamber in step 12 for stress relief. In another embodiment the material is treated for straightening. In a further embodiment, the material is treated for the application of a final insulative coating thereto. In still another embodiment, the material is treated under tension. In yet another embodiment, step 12 is conducted in yet another embodiment in a protective atmosphere. In still a further embodiment, the protective atmosphere is in an atmosphere which is substantially unreactive with iron, typically a nitrogen-rich atmosphere. In another embodiment, the material is treated for about 1 to 2 minutes. In a further embodiment, the material is treated at a temperature of from about 650 to 900 degrees C. The product of step 12 in one embodiment is Grain Oriented Electrical Steel. In a further embodiment, the material from step 12 can be treated in an additional step, namely 12.a, such that the exposed surface is heated by a focused laser to create an incision. In an embodiment, the laser source is capable of reaching at least 1200 degrees C. In a further embodiment, the incision is created in no more than about 100 milliseconds. In one embodiment, the incision is from about 0.01 to 0.02 mm deep. The product of step 12.a of still another embodiment, is Laser Scribed Grain Oriented Electrical Steel. 

1. A process for making high grades of specialty steels commonly known as electrical steels which comprises: a. providing a molten steel feed; b. performing a chemical analysis of the molten steel which includes identifying which chemical components of the steel feed should be removed by conducting a chemical reaction and/or by adding a chemical component to obtain the desired chemical analysis, and then removing the requisite chemical components from the molten steel and/or adding the requisite chemicals to the molten steel; c. casting a steel slab from the molten steel feed having a thickness of from about 1 mm to 200 mm; d. reheating the cast slab to a temperature between 1150 degrees C. to 1400 degrees C.; e. reducing the thickness of the cast slab to a thickness of from about 1.0 to 2.5 mm to form a hot band; f. cooling the hot band to a temperature of about 550 degrees C.; g. annealing and normalizing the hot band from step f. in a protective atmosphere of nitrogen; h. pickling the annealed and normalized hot band to remove scale formed thereon with up to 5% by weight of SiO₂; i. reducing the thickness of the hot band from step h. to up to about 0.65 mm; j. decarbonizing and annealing the hot band from step i. in a protective atmosphere of N₂; k. reducing the thickness of the hot band from step j. to up to about 0.63 mm; l. coating the hot band from step k. with magnesia for preventing sticking in step m. and for forming an initial insulative coating; m. annealing of the coated hot band to form large grains and to develop a secondary insulative coating; n. treating the annealed and coated hot band for stress relief in a protective atmosphere which is substantially non-reactive with iron and which applies a final insulative coating thereto; and o. straightening the hot band from step n. and forming grain oriented electric steel.
 2. Process according to claim 1, which further comprises treating the hot band from step j. in a chamber such that its surface is exposed to liquefied ammonia thereby forming high induction electric steel.
 3. Process according to claim 1, which further comprises treating the grain oriented steel from step n. such that the exposed surface is heated by a focused laser to create an incision therein and thereby forming laser scribed grain oriented electrical steel.
 4. Process according to claim 2, which further comprises treating the high induction electric steel from step n. such that the exposed surface is heated by a focused laser to create an incision therein and thereby forming laser scribed high induction electrical steel.
 5. Process according to claim 1, which further comprises a. reducing the thickness of the hot band from step h. to up to about 0.63 mm; b. forming non-oriented grains within the reduced thickness hot band; and c. applying an insulative coating to the hot band including non-oriented grains and forming non-oriented electrical steel.
 6. Process according to claim 1, wherein annealing in step m. is conducted at a temperature of at least about 1200 degrees C.
 7. Process according to claim 1, wherein annealing in step m. is conducted for at least about 130 hours.
 8. Process according to claim 1, wherein the protective atmosphere which is substantially non-reactive with iron in step n. is a nitrogen-rich atmosphere.
 9. Process according to claim 1, wherein the final protective coating in step n. is applied under tension.
 10. Process according to claim 1, wherein the second insulative coating in step m. is substantially non-conductive.
 11. Process according to claim 1, wherein in step a., the steel is melted at a temperature of about 1500 degrees C.
 12. Process according to claim 1, wherein in step b., the chemical composition of the molten steel feed is adjusted so that the amount of Aluminum is up to about 0.5% by weight, the amount of Nitrogen is up to about 0.05% by weight, the amount of Manganese is up to about 0.3% by weight, and the amount of Silicon is up to about 5%, by weight, based on the total weight of the molten steel feed.
 13. Process according to claim 1, wherein in step f. initially cooling the hot band is with ambient air.
 14. Process according to claim 1, wherein in step f. initially cooling the hot band with is ambient air and then the initially cooled hot band is rapidly cooled with water.
 15. Process according to claim 1, wherein in step h. removing from about 20 to 30 g/m² of the scale formed on the hot band.
 16. Process according to claim 1, wherein in step i. reducing the thickness to up to about 0.63 mm and in step k. reducing the thickness to up to about 0.60 mm.
 17. Process according to claim 2, wherein the hot band from step j. is treated with activated ammonia.
 18. Process according to claim 1, wherein in step g. annealing and normalizing in the protective atmosphere of nitrogen is conducted in a H₂O/H₂ atmosphere.
 19. Process according to claim 18, wherein the H₂O/H₂ ratio is from about 0.05 to 0.95.
 20. A process for making high grades of specialty steels commonly known as electrical steels which comprises: a. providing a molten steel feed; b. performing a chemical analysis of the molten steel which includes identifying which chemical components of the steel feed should be removed by conducting a chemical reaction and/or by adding a chemical component to obtain the desired chemical analysis, and then removing the requisite chemical components from the molten steel and/or adding the requisite chemicals to the molten steel; c. casting a steel slab from the molten steel feed having a thickness of from about 1 mm to 7 mm and forming a hot band; d. cooling the hot band to a temperature of about 550 degrees C.; e. annealing and normalizing the hot band from step f. in a protective atmosphere of nitrogen; f. pickling the annealed and normalized hot band to remove scale formed thereon with up to 5% by weight of SiO₂; g. reducing the thickness of the hot band from step h. to up to about 0.65 mm; h. decarbonizing and annealing the hot band from step i. in a protective atmosphere of N₂; i. reducing the thickness of the hot band from step j. to up to about 0.63 mm; j. coating the hot band from step k. with magnesia for preventing sticking in step m. and for forming an initial insulative coating; k. annealing of the coated hot band to form large grains and to develop a secondary insulative coating; l. treating the annealed and coated hot band for stress relief in a protective atmosphere which is substantially non-reactive with iron which applies a final insulative coating thereto; and m. straightening the hot band from step n. and forming grain oriented electric steel. 